Vehicle steering is generally controlled by a driver hand wheel that directs the angle of the vehicle wheels used for steering. The movements of the driver hand wheel are transmitted to the vehicle wheels by mechanical linkages and/or electronic components. The vehicle road wheels that change angle are located in the front of the vehicle in a system referred to as “front steering”. The angle of the road wheels is referred to as “road wheel angle”.
Active front steering (AFS) is a term referring to the use of electronic components to actively control or assist the steering of a vehicle so as to enhance steering performance beyond that possible by only direct mechanical linkages. There are many possible ways to enhance steering performance; for example, steering can be adapted to the weather conditions, to the behavior and habits of the driver, to provide orderly stopping if the driver loses control.
At higher speeds, large changes in the angle of the vehicle wheels can cause undesirable shifts in vehicle direction. Accordingly, precise driver control at high speeds requires subtle changes in the angle of the driver hand wheel. At low and medium speeds, a vehicle generally will be steered into tighter or larger angle turns for parking or manipulating corners. Large turns of the driver hand wheel are usually necessary to make large turns of the vehicle wheels. Driving is easier if the vehicle wheels turn less for corresponding driver hand wheel turns at high speed and more for corresponding driver hand wheel turns at low speed.
In an AFS system, variable gear ratio (VGR) steering is a method for adding and subtracting steering angle to the target road wheel angle implied by the driver's hand wheel input. This can be accomplished by mechanical or electrical components. It is desirable to insure that the VGR system is fail-safe, operates in a safe manner, and does not vary greatly from its intended operational parameters.
Open loop control refers to the operation of a road wheel angle controller without feedback and independent of any supervisory control system. Lead steer is a method of anticipating the driver's intent at the hand wheel that may be implemented in an automotive control module. The AFS system may combine VGR and lead steer to ascertain the target road wheel angle for open loop control.
Closed loop control or electronic closed loop control (ECS) refers to the operation of a road wheel angle controller with feedback from a supervisory control system. For closed loop control, an angle offset from a supervisory control system is added to the VGR steering angle to determine a target road wheel angle that should be implemented by the steering mechanism.
Various types of microcontroller or microprocessor-based controllers found on many conventional vehicles include a supervisory control module (SCM), a hand wheel sensor (HWS) module and an AFS module. Such AFS system modules are typically implemented with any one of numerous types of microprocessors, microcontrollers or other control devices that appropriately receive data from one or more sensors or other sources, process the data to create suitable output signals, and provide the output signals to control actuators, dashboard indicators and/or other data responders as appropriate.
The various components of a vehicle-based control system typically inter-communicate with each other and/or with sensors, actuators and the like across any one of numerous types of serial and/or parallel data links. Today, data processing components within a vehicle are commonly interlinked by a data communication network such as a Controller Area Network (CAN). An example of a CAN is described in ISO Standard 11898-1 (2003).
When there is an error in an AFS system module, security metrics usually call for the locking of the steering actuators within a designated period of time. However, message traffic on the CAN may cause communication delays that prevent timely error detection and/or timely actuator locking. In this regard, the AFS system may not detect errors in a timely manner to lock the AFS actuator within the time allowed by the security metrics. Thus, it is desirable to have an AFS system and method that lowers the time to lock the AFS actuator within the time allowed by the security metrics, reduces the probability of false failure detection, and accurately detects actual failures during steering control.
Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.